In Accounts and Billing, Billing, I see the invoice number and click on the number/link. A popup box tells me I need to install Acrobat 5.0 or above in order to use this feature.
Your application is not even giving me a PDF to process (view/download) and so there's no error message or failure that I can tell you about. I would expect your application to offer me a dialog box asking me for the folder where I want to have the PDF placed. That's the most common method I see on other sites, but some will load the PDF into a window (and therefore let me print it or save it).
I'm using OSX 10.4.11 (Tiger) and the browser is Firefox 3.5.2. You don't support Safari 4.0.2, Camino or Opera as I've tried them all. In a few days time, I'll update this thread if I find that OSX 10.6 (Snow Leopard) makes any difference to the way your application works.
Thanks for following this up.